Television and like camera tubes



Sept. 11, 1962 E. D. HENDRY 3,054,019

TELEVISION AND LIKE CAMERA TUBES Filed June 4, 1959 JNVENTQR ATTORNEYS Unite 3,054,019 Patented Sept. 11, 1962 3,054,019 TELEVISION AND LIKE CAMERA TUBES Eric Douglas Hendry, Chelmsford, England, assignor to English Electric Valve Company Limited, London,

England, a British company Fiied June 4, 1959, Ser. No. 818,186 Claims priority, application Great Britain Aug. 29, 1958 4 Claims. (til. 315-11) This invention relates to the television and like camera tubes generally known as image orthicon camera tubes. More specifically the invention relates to image orthicon camera tubes of the kind in which, in order to ensure that the electron beam scanning the target shall always be incident thereon at right angles thereto in all positions of cathode ray deflection, there is provided an electrostatic field-terminating mesh which is mounted substantially parallel to and adjacent the target on the gun side thereof and is electrically connected to the normally provided internal wall coating on the inside of the tube envelope. This field-terminating mesh is for the purpose of correcting the electro-static field in the neighbourhood of the target and increasing the strength of the decelerating field in front of the target. This reduces or eliminates distortion due to deflection of the ray by the electrostatic field due to the charge pattern stored on the target.

Certain operating defects have been found to occur with' image orthicon camera tubes of the kind referred to, due to secondary electrons from the field-terminating mesh. One such defect is that such secondary electrons released from the mesh as a result of bombardment by electrons of the ray on its way to the target causes an undesirably high DC. output. Also a distorting out-ofphase signal occurs due to secondary electrons released from the mesh returning to the electron multiplier section of the tube. Since these electrons start from the mesh with very low velocity they will arrive at the input of the multiplier section a little later than (i.e. a little displaced in time with respect to) the main signal; in other words they cause an out-of-phase and therefore a distorting signal component to be included in the output from the tube.

The present invention seeks to provide improved image orthicon camera tubes of the kind referred to wherein the foregoing defects are substantially reduced or eliminated without having to make any but minor and easily made structural changes in the tube and without requiring the application of any inconvenient additional biasing potentials to electrodes of the tube. Indeed, in its preferred embodiments, the invention achieves its object by providing merely a single additional electrode in a position which is quite convenient from the structural point of view and connecting that electrode permanently and internally to the cathode of the gun of the tube.

According to this invention an image orthicon tube of the kind referred to is provided with means for establishing, in the neighbourhood of the entrance to the electron multiplier section of the tube, a potential surface which is negative with respect to the potential of the fieldterminating mesh adjacent the target structure of the tube, whereby secondary electrons emitted by said mesh are substantially prevented from entering said multiplier section.

Preferably the means for establishing the potential surface are constituted by a ring electrode '(or its equivalent, such as a number of inter-connected electrodes in a ring) mounted on the normally provided electron gun and multiplier section structure of the tube at the end of said structure nearer the target, and means for applying a predetermined operating potential to said ring electrode. In practice said predetermined operating po- CTl tential may be gun cathode potential in which case the ring electrode (or its equivalent) may be simply con nected, internally of the tube, to the gun cathode.

These and various other objects and features of the invention will be more clearly understood from a reading of the detailed description of the invention in conjunction with the drawing in which:

FIG. 1 is a view in elevation, partly in section, of one illustrative embodiment of this invention; and

FIG. 2 is a plan view of one embodiment of the annular electrode.

Referring to FIG. 1 of the drawing, an image orthicon tube of the kind referred to has an envelope 1 with a photo-cathode 2 on the inside of the end wall thereof. 3 represents the usual internal coating on the inner wall of the tube. Connecting to this coating is a fieldterminating mesh 4 which is adjacent and parallel to the customary target structure conventionally represented at 5. The electron gun is at the end of the neck of the tube, its cathode being given the reference 6. Arranged around the gun, as in the usual way, is a secondary electron multiplier section comprising a number of dynodes conventionally represented at 7. The electron gun and electron multiplier section are parts of a single mechanical unit or structure as in the customary way. As so far described the tube is as Well known per se and is typical of a known image orthicon of the kind referred to. In accordance with this invention, however, there is provided an additional annular electrode 8 adjacent to the entrance of the multiplier section comprising the dynodes '7. The electrode 8 is simply mounted on the structural unit comprising the gun and multiplier section and on the end of said unit nearer the target, the said electrode 8 being in the neighbourhood of the entrance to the multiplier section. In use the electrode 8 is maintained at such potential that secondary electrons from the mesh are prevented from entering the multiplier section. The electrode 8 may be provided wih an external connection by which it may be biased to a suitable potential, but in practice it is not necessary even to do this, and good results can be obtained by merely connecting the said electrode 8 internally of the tube to the cathode 6 of the gun. This is indicated in FIG. 1 of the drawing by the connection 9.

Another embodiment of the annular ring 8 of FIG. 1 is shown in FIG. 2 in which ring 8' is segmented and the segments are interconnected by conductors 10.

The only operating change required for a tube as described and illustrated as compared to an otherwise similar known tube is that the voltage applied to the Wall coating 3 has usually to be a little higher than in the case of a known tube in order to achieve proper beam focusing.

I claim:

1. An image orthicon tube comprising an electron gun having a cathode, a target, an internal wall coating and an electrostatic field-terminating mesh mounted substantially parallel to and adjacent the target on the gun side thereof, said mesh being electrically connected to said internal wall coating to insure that the electron beam scanning the target shall always be incident thereon at right angles thereto in all positions of cathode ray deflection, said tube further comprising an electron multiplier section structure surrounding said electron gun, an annular electrode mounted adjacent said structure on the side of said field-terminating mesh, and means for applying a predetermined operating potential to said annular electrode for establishing across the entrance to the electron multiplier section of the tube, a potential 0 surface which is negative with respect to the potential of the field-terminating mesh, whereby secondary electrons emitted by said mesh are substantially prevented from entering said multiplier section.

2. An image orthicon tube as set forth in claim 1 wherein said predetermined operating potential is the electron gun cathode potential and said annular electrode is connected, internally of the tube, to the electron gun cathode.

3. An image orthicon tube comprising a target, electron gun means for providing an electron beam of substantially zero velocity at said target, an electrostatic field terminating mesh substantially parallel to and adjacent the target on the gun side thereof and means for applying potential to said mesh to ensure that the electron beam scanning the target shall always be incident thereon at right angles thereto in all positions of cathode ray deflection, said tube further comprising an electron multiplier section encircling said gun means, an annular electrode encircling said multiplier section and mounted between said section and said mesh and means for applying operating potential to said annular electrode thereby to establish across the entrance to the electron multiplier section of the tube a potential surface which is negative with respect to the potential of the field terminating mesh, whereby secondary electrons emitted by said mesh are substantially prevented from entering said multiplier section.

4. An image orthicon tube as set forth in claim 3, wherein said annular electrode is composed of a number of inter-connected electrodes in a ring.

References Cited in the file of this patent UNITED STATES PATENTS 2,652,515 McGee Sept. 15, 1953 2,747,131 Sheldon May 22, 1956 2,749,463 Pierce June 5, 1956 

